A cat that gains 10 percent of its body weight in 4 weeks has not simply eaten more. On a 4.5 kg cat that is 450 g, and part of it can be fluid, because 1 liter of free abdominal fluid weighs close to 1 kg. Weigh weekly on a 5 g scale, measure girth at the umbilicus, and let the 4 week trend, not a single reading, decide what happens next.
Two numbers separate a food problem from a medical one: 10 percent in 4 weeks, and a girth increase of 2 cm in 7 days. The first is the workup threshold; the second is the fluid red flag.
Why 4 weeks and 10 percent is the line
Fat is expensive to build. One kilogram of body fat stores roughly 7,700 kcal, so 450 g of true fat gain needs a surplus of about 3,500 kcal, which is 125 kcal a day for 28 days. A 4.5 kg cat at maintenance needs 200 to 250 kcal a day; gaining 450 g of fat in 4 weeks means eating 325 to 375 kcal a day, a 30 to 50 percent overshoot. That is possible with free feeding 80 g of a 400 kcal per 100 g dry food, which delivers 320 kcal before treats. Normal winter coat gain runs 1 to 3 percent over 8 to 12 weeks, which is 45 to 135 g on the same cat, not 450 g. Anything faster than 3 percent in 4 weeks has a cause worth naming.
Fat or fluid: 3 checks that take 10 minutes
| Week | Weight (g) | Change from week 1 | Girth at umbilicus (cm) | Body condition |
|---|---|---|---|---|
| Week 1 | 4500 | 0 g | 38 | 5 of 9 |
| Week 2 | 4620 | +120 g | 39 | 5 of 9 |
| Week 3 | 4785 | +285 g | 40 | 6 of 9 |
| Week 4 | 4960 | +460 g | 42 | 6 of 9 |
| Week 5 | 5105 | +605 g | 44 | 6 of 9 |
Check 1 is the shape: fat sits over the ribs and hips and the belly hangs in a pouch, while fluid makes a symmetric, tight, drum like abdomen that is wider at the flanks. Check 2 is the feel: ribs under 1 cm of fat are a 5 of 9; ribs you cannot find under 2 cm are a 7 of 9. Check 3 is the fluid wave, which only a vet should test, because a firm push on a tense abdomen in a cat with heart disease can cause distress. A cat that gains 605 g while eating the same 60 g of food has fluid or a mass, not a calorie problem.
What fluid in the abdomen means
Free fluid in a cat is never normal. The common causes are the wet form of FIP, congestive heart failure, liver disease, neoplasia and peritonitis, and they are separated by tapping 5 to 10 mL of fluid and sending it for protein and cell counts. Ultrasound is the fastest way to see 20 to 50 mL of fluid that a hand cannot feel, and it also shows an enlarged liver or thickened bowel. A cat with ascites and a resting respiratory rate of 30 or more breaths per minute is at risk of pressure on the diaphragm and needs a same day visit, not a diet.
Hypothyroidism is uncommon in cats, and still worth ruling out
Most thyroid disease in cats is the opposite problem: hyperthyroidism affects roughly 1 to 4 percent of cats over 10 and causes weight loss with a good appetite. Hypothyroidism is uncommon, and most cases are iatrogenic after radioiodine or a thyroidectomy overshoots, with a smaller number congenital or central. The picture is weight gain without an appetite increase, 2 to 3 hours more sleep a day, cold seeking, dry coat, and a heart rate under 140 beats per minute where 160 to 220 is normal. A total T4 below 1.0 microgram per dL supports the diagnosis, and a free T4 by dialysis confirms it. Also rule out pregnancy, which runs 63 to 65 days and adds 1 kg across 2 to 4 kittens, and simple free feeding in a house with 3 or more cats.
Calorie accounting in 3 steps
Step 1: weigh 7 days of food in grams and convert with the label energy density, because a 400 kcal per 100 g dry food and a 75 kcal per 100 g wet food differ by 5 times. Step 2: compute resting energy as 70 times body weight in kg to the power 0.75, which is 216 kcal for 4.5 kg, then multiply by 1.2 to 1.4 for a neutered indoor cat, giving 260 to 300 kcal. Step 3: compare actual intake with that target. If intake is 100 kcal over target, cut 10 percent of the ration for 14 days, reweigh, and adjust again by 10 percent; a safe loss is 0.5 to 1 percent of body weight per week, so 25 to 45 g a week on this cat.
Weighing with 5 g precision
Use a kitchen scale that reads 1 g increments, or a baby scale at 5 g, on a hard floor, at the same time each morning before breakfast and after the litter box. Take 3 readings and use the average, because a cat shifts 20 to 40 g between positions. Log grams, not pounds, and keep 12 weeks of data so a 120 g rise in week 3 does not get lost in noise. A 5 g resolution catches 0.1 percent of body weight on a 4.5 kg cat, which is the resolution needed to see a trend inside 2 weeks.
When to see a vet
Within 24 hours: 10 percent weight gain in 4 weeks, a 2 cm girth increase in 7 days, weight gain with no change in appetite, or 3 weeks of a steadily growing belly. Same day: a distended abdomen with 30 or more breaths per minute, pale gums, or a cat that stops eating for 24 hours on top of the gain. Emergency: sudden collapse or hind-limb weakness (collapse guide), open-mouth breathing, or a cat that cannot settle and cries when the belly is touched. Bring the 12 week weight table and the girth measurements. A cat that is losing weight while eating normally is a different workup (weight loss with normal appetite), and any suspected exposure to lilies or permethrin belongs in the toxicity checker before a diet change.
Five steps for the next 30 days
- Weigh the cat today on a 5 g scale and start the table.
- Measure girth at the umbilicus with a soft tape every 7 days.
- Weigh 7 days of food and compute the daily kcal with the label density.
- Book a visit if the 4 week change reaches 10 percent.
- Photograph the belly from the same angle weekly, next to a ruler.
Terms mentioned in this article
Frequently asked
Is 200 g of weight gain in a week normal?
On a 4.5 kg cat, 200 g is 4.4 percent, which is fast for 7 days and always worth a second weighing. Weigh 3 times at the same hour, check girth and ribs, and look at the food log. If the next week adds another 150 g or more, or the belly is tight rather than soft, book a visit for fluid and thyroid testing.
How do I tell fluid from fat at home?
Fat is soft, sits over the ribs and hangs in a pouch, and the cat still has a waist behind the ribs. Fluid makes a symmetric, tense abdomen that is wide at the flanks, and the cat may breathe 30 or more times a minute at rest. A 2 cm girth increase in 7 days with a flat appetite is fluid until a vet says otherwise.
Can hypothyroidism make a cat gain weight?
It can, but it is uncommon in cats, and most cases follow radioiodine or thyroid surgery that overshot. Expect weight gain without an appetite increase, more sleeping, cold seeking and a heart rate under 140. A total T4 under 1.0 microgram per dL plus a free T4 by dialysis confirms it; hyperthyroidism is far more common and causes weight loss instead.